Your pop3 software will be probably listed in /etc/inetd.conf (if you're using inetd). Run
cat /etc/inetd.conf|grep pop-3
you'll get a line where your pop3 software is listed. One of the elements will be path (and filename) of your pop3 software.
Browse your logs (especially in /var/log/mail), it should be shown when someone's using pop3.